This invention concerns a capacitive acceleration sensor, in particular for use in motor vehicles, complete with planar build-up and a movable self-supporting structure, located inside a hollow space, for changing capacity,
wherein the acceleration sensor comprises two semiconductor elements,
wherein at least one of these two semiconductor elements features at least one cavity of a first type,
wherein at least one of these two semiconductor elements features at least one cavity of a second type, within which a self-supporting structure will be located and bonded to the semiconductor element such that this self-supporting structure will be freely movable in a vertical direction relative to the surface of the two semiconductor elements,
and wherein both semiconductor elements are bonded with their surfaces and located such that a cavity of the first type from the one semiconductor element and a cavity of the second type from the other semiconductor element together form a hollow space.
Capacitive acceleration sensors are used, for example, in a motor vehicle for its suspension control or as impact sensors for triggering protection devices such as airbags.
In this connection, sensors are known which function according to the piezoelectric or piezoresistive principle. However, test arrangements according to the piezoelectric principle require a very high input impedance in order to be able to evaluate the relatively weak signal of a piezoelectric sensor, and therefore are sensitive--for example--to electrostatic interference. In contrast to test arrangements according to the piezoresistive principle, this arrangement according to the piezoelectric principle cannot be used to measure static or low frequency acceleration events. Piezoresistive sensors are very temperature dependent as regards their sensitivity and offset voltage; in addition, their manufacture is complex and expensive.
Capacitive acceleration sensors, however, feature several advantages which makes them particularly suitable for use in motor vehicles. Thus, they are able to detect static and low frequency accelerations; secondly, they are comparatively insensitive to temperature variations; and, thirdly, their so-called expansion factor is greater than that of piezoresistive sensors, entailing a higher sensitivity or greater measurement range.
So-called bulk acceleration sensors are known as capacitive acceleration sensors; however, these are manufactured by means of a very work-intensive process. Moreover, to produce one such bulk acceleration sensor, three wafers will usually be required; therefore, acceleration sensors of this type are relatively expensive due to high material and manufacturing costs, which does nothing to promote their use, e.g., in compact motor vehicles as sensors for triggering an airbag system.